1. Field of the Invention
The present invention relates generally to the art of establishing the three-dimensional positions of a set of points on a body with contactless measuring means, and more particularly to a system for determining the deviation of a set of points from a set of reference points in a three-dimensional cartesian coordinates system. Once these positions have been determined, they are either numerically or graphically compared to a set of reference points. Even more particularly, the system may be used in conjunction with motor vehicle repair and maintenance, for example body or frame straightening and repair or wheel alignment, to compare the actual position of certain points on the vehicle to the manufacturer's specifications for that vehicle.
2. Description of the Related Art
It is frequently necessary to know the actual position of a point on a body relative to the desired position of such a point. This is particularly true with regard to motor vehicle repair and maintenance. A system that could detect a discrete set of points on a vehicle body or frame element and compare the actual positions of those points to their desired positions would be helpful in such operations as body repair, frame straightening and wheel alignment.
In particular, in detecting and correcting defects in a vehicle frame, a straightening rack is often used. For example, such a rack may consist of chains, cables or other means attached to hydraulic cylinders and to the vehicle frame to push and/or pull the frame back into its proper configuration. Examples of such devices are shown in U.S. Pat. No. 3,590,623 issued to Hunnicutt et al. on July 6, 1971, and reissued U.S. Pat. No. Re. 31,000, issued to LeGrand et al. on July 27, 1982.
Examples of wheel alignment systems that would benefit from incorporating the present system are U.S. Pat. No. 3,793,736 issued to Cufrini on Feb. 26, 1974, U.S. Pat. No. 4,097,157 issued to Lill on June 27, 1978 and U.S. Pat. No. 4,344,234 issued to Lill et al. on Aug. 17, 1982.
With respect to means used to acquire measurement data there are no such systems known to applicant in the auto body and wheel alignment art which employ acoustic measuring techniques. However, many methods and means have been disclosed in prior patents for distance measurement. A number of such devices require direct physical contact between the measuring means and the point whose position is to be determined. Several of these devices mechanically measure the position being touched by a probe, as in U.S. Pat. No. 4,536,962 issued to Hense et al. on Aug. 27, 1985 and U.S. Pat. No. 4,549,359 also issued to Hense et al. on Oct. 29, 1985. Other devices require physical contact to provide a conductive path for a travelling signal. In U.S. Pat. Nos. 4,035,762 and 4,231,260 issued to Chamuel on July 12, 1977 and Nov. 4, 1980, respectively, a delay element acts as the medium for a measuring signal. The position of the measured point is determined by measuring the phase shift in the travelling signal.
These devices all suffer the same shortcomings. If readings are to be taken more than once, when straightening an auto frame, for example, the delay element or other position sensor must be positioned identically a number of times. In addition to the potential inaccuracy, it is time consuming to have to reposition the element or sensor for every point each time a new reading is to be taken. A device that avoids these problems would be an important improvement.
Several devices incorporating contactless measuring means have been developed. One such device is described in U.S. Pat. No. 3,176,263 issued to Douglas on Mar. 30, 1965. Douglas generally shows a drape of small explosives over the body of the object to be measured. Surrounding the area of the body are a number of microphones. The small explosive charges are detonated and the response times measured by the microphones. By compiling and processing the times measured by the microphones, the general shape of the body and its proportions can be measured and recorded. The system as disclosed by Douglas would be impractical for purposes of measuring and recording positions on an auto body or frame since the explosions would, no doubt, have an adverse effect on the paint and structure of the body. In addition, a new drape of explosive charges would be required for each reading, which would be totally impractical.
Another contactless measuring device is shown in U.S. Pat. No. 3,731,273 issued to Hunt on May 1, 1973. The Hunt patent shows a mechanical triggered spark gap which is contained in a probe shown in FIG. 5 of Hunt. To measure a a given position, one places the spark gap at the tip of the probe at the point to be measured. By applying pressure to the probe, physical contact between electrical leads is made allowing a spark to be generated. The travel time of the acoustic wave is measured by two microphones and the position calculated. Several problems are encountered with the Hunt device, However. First, the spark gap must be mechanically and physically triggered. This means applying pressure to the probe which may dislocate the probe a slight distance. In a system measuring small distances, such as applicants' system, such dislocation could easily be greater than the accuracy of the device. Second, the device shown in Hunt requires that the spark gap be located at the position to be measured. Therefore, a point which is inaccessible to the probe's spark gap or which is not able to be accurately measured by such a configuration, cannot be measured by the device shown in Hunt. Finally, Hunt suffers from one other deficiency. If a number of measurements are to be taken at the same point while the body measured is moving or changing shape, the Hunt device does not provide for a consistent and accurate means of measuring the identical point a number of times.
U.S. Pat. No. 3,821,469 issued to Whetstone et al. On June 28, 1974 shows another device for measuring the position of a point in space. Whetstone uses a stylus similar to the probe found in Hunt and a series of orthogonally positioned receptors. The device shown in Whetstone requires that the receptors define the entire space throughout which the stylus moves. This obviously is an impractical restriction on the device if it is to be used to measure along the length, width and depth of an automobile or truck body or frame.
U.S. Pat. No. 3,924,450 issued to Uchiyama et al. on Dec. 9, 1975 also shows a device for measuring three-dimensional coordinates. The device shown uses a supersonic oscillator to generate a signal to be timed. The signal is generated at a point P and is received at at least three points, A, B and C. Uchiyama does not disclose the means or method for converting or for measuring the travel time of a continuous supersonic wave. The known methods for accomplishing this suffer from the same shortcoming. The accuracy available with such a system is extremely poor when compared with the digital systems used in applicants' device. Because the device disclosed is used for measuring models of large scale operations, such as marine engine rooms and landbase plants, the accuracy is not as important and, therefore, the high resolution required in applicants' device is not considered important in the area of art addressed by Uchiyama.
In U.S. Pat. No. 3,937,067 issued issued to Flambard et al. on Feb. 10, 1976, a device is disclosed that is used to measure angular displacements. Flambard uses the reflective properties of an ultrasonic wave to measure displacement. This technique is naturally not desirable, applicable or practical in applicants' system where any reflection will only distort the measurement of the travel time.
Another patent showing a distance measuring scheme is U.S. Pat. No. 4,276,622 issued to Dammeyer on June 30, 1981. Dammeyer generally shows a circuit used to measure the distance between an ultrasonic transmitter and an ultrasonic receiver. The transmitter generates an ultrasonic energy burst in response to an energizing signal. The receiver receives the ultrasonic burst and generates a detection signal in response thereto. While the ultrasonic signal is in transit, a ramp generator is activated, allowing a capacitor to linearly charge for a period of time. The distance the signal travelled is therefore in direct proportion to the accumulated voltage potential of the capacitor, in this case capacitor C5 in FIG. 4. The rate of potential increase is controlled by adjusting resistor R10. The method used by Dammeyer, while providing a coarse measurement of distance, suffers, as does uchiyama, from the fact that the analog signals used are only a rough approximation when compared to those available with digital circuitry and suffer from both time and temperature dependency. Therefore, while the measured potential of capacitor C5 is representative generally of the distance covered by the ultrasonic signal, it does not approach the accuracy and resolution possible with the digital circuit and software employed by applicants in their invention.
Finally, U.S. Pat. No. 4,357,672 issued to Howells et al. on Nov. 2, 1982, discloses another distance measuring apparatus using acoustic signals. During the transit time of an acoustic signal, a microprocessor counts the number of instruction cycles it executes, thereby generating a count which is generally indicative of the amount of time the acoustic signal takes to travel from the stylus to the microphone. In the claims and specification, however, Howells specifically states that the timing mechanism will be the internal instruction count of the microprocessor. He states that no additional clock or scaler is necessary to operate the system. He thus limits the accuracy and resolution of the system by limiting the timing frequency to the execution timing of instruction cycles.
There are a number of other acoustical devices which may be used to detect defects in various objects. These devices base their calculations on different arrival times of a signal reflected off of a defect in an object. Therefore, many of the principles used to construct and use such devices are inapplicable to a system in which no reflection is desired and a homogeneous transit medium is required. Examples of such devices include U.S. Pat. No. 3,875,381 issued to Wingfield, deceased et al. on Apr. 1, 1975; U.S. Pat. No. 4,096,755 issued to Hause et al. on June 37, 1978; and U.S. Pat. No. 4,523,468 issued to Derkacs et al. on June 18, 1985.